2.9- How do you keyframe an object's render visibility?
I've seen the answer to this question in this tutorial Blender 2.83 : Grab &amp; Release Weapons (In 60 Seconds!!!) by Royal Skies LLC a while ago.
You can keyframe the object's visibility in the Object Properties, section Visibility. There are 2 checkboxes to control it: Show in Renders and ... Viewports.